The need for sensitive magnetic microscopes with a high degree of spatial resolution is felt in many industries. For example, IC engineers and designers can use such a microscope to carry out non-invasive and non-destructive measurements of electrical current distributions within an IC chip, and they may survey the operation of tiny devices—pinpointing electrical and structural defects down to the smallest unit (transistor, diode, interconnects, vias, etc.). They may also study electro-migration of interconnects to develop even finer conducting lines. In another application, a data-storage professional can image ultrafine domain structures of future recording media with a magnetic microscope of sufficient sensitivity and resolution. These media will have an increasingly small domain size, eventually approaching the adverse superparamagnetic limit. Furthermore, the magnetic microscope is also applicable to a range of basic research areas, such as flux line structure investigation.
Many physical objects generate magnetic fields (H) near the objects' surfaces, and the magnetic microscope can obtain images of the magnetic fields by scanning a magnetic sensor on the surface of the object of interest. Such images can be spatially microscopic and weak in field strength. Nevertheless, these images reveal important signatures of inherent electrical and magnetic processes within the objects. For example, the magnetic image of a magnetic thin film discloses its internal magnetic domain structure (spatial electron-spin configuration). The electrical currents within an integrated circuit (IC) chip generate external magnetic images, which not only contain information of current-distribution, but also the frequencies with which various components on a chip are operating. A type II superconductor also creates an image of threading magnetic flux lines, whose structure and dynamics are fundamental properties. Researchers have confirmed the d-wave symmetry in high-Tc superconductors by studying the flux line images in a uniquely designed sample.
There are currently a number of techniques for imaging magnetic fields at surfaces.
Electron holography and SEMPA (scanning electron microscopy with polarization analysis) require high vacuum operation and delicate sample preparation. Both techniques offer static field images with good spatial resolution. However, these instruments are expensive and demand great technical skill to operate.
The magneto-optical microscope is a relatively simple system and is suitable for time-resolved imaging. However its field sensitivity and spatial resolution are poor.
The conventional scanning magnetic microscope has a microscopic field sensor, typically a superconducting quantum interference device (SQUID), a Hall probe, or simply a magnetic tip. This type of microscope scans the magnetic sensor relative to a sample to obtain a local field image. Though very sensitive, a SQUID probe is poor in resolution (˜5 μm), and requires cryogenics (77K). A Hall probe can operate under ambient conditions, but its sensitivity in the prior art is low. The magnetic microscope equipped with a magnetic tip can only measure the gradient of the magnetic field, and cannot sense a high frequency signal (e.g. MHz-GHz).
Prior to this invention the conventional magnetic imaging systems, such as the conventional scanning magnetic microscope, suffered from at least one of an inadequate sensitivity or an inadequate spatial resolution, resulting in the generation of magnetic field images that were less than optimum.